The speed of a Ward Leonard DC drive is controlled by a generator field current Igf. Therefore, the performance of speed control depends on the quality of field current Igf control. To achieve slow speed, the generator field current Igf has to be controlled at a very low level, and during deceleration and a consequent reversal, the field current Igf has to cross zero and change its sign repsectively.
A controlled rectifier using thyristors is a cost effective type of regulating unit for the generator field current Igf control. FIG. 1 shows a typical thyristor configuration, a two-way power converter, as a generator field power converter 5 in a cascade drive control scheme. A velocity controller 2 compares actual velocity with a dictated velocity and sets an appropriate torque command T which is the reference input for an armature current control loop 3. An armature current regulator 4 calculates a firing angle F which activates the SCR converter 5 to set the output voltage Vgf of a generator field winding 7 of a generator 8 of an MG-set 9, thus providing a current Iscr through a generator field winding 7. The SCR converter 5 is connected to a power transformer secondary winding 10, part of a typical sinusoidal voltage source (not shown), for receiving a power signal. A DC hoist motor 11 will be activated by the generator armature current to produce a torque proportional to the armature current Ia, thereby moving a counterweight 12 and a car 13 through rotation of a sheave 14. Actual velocity is fed back to be summed with the dictated velocity by means of a tachometer 15. A digital synchronization signal 9 is generated by 16, in phase with the line voltage, across the power transformer secondary winding 10.
The converter 5 is a two-way power converter consisting of two center tap-connected pairs of thyristors (T1,T3) and (T2,T4) in anti-parallel connection. The thyristors (T3) and (T4) are fired to achieve a generator field current Igf in the positive direction, causing a motor torque for the elevator traveling in the up direction, while (T1) and (T2) are fired to achieve current in the other direction, causing a motor torque while the elevator is traveling in the down direction. The thyristors for the positive half-wave of the line voltage are (T2,T3), and the thyristors for the negative half-wave of the line voltage are (T1,T4). The gates 17 of the SCRs 18 are connected to the current regulator 4 which controls the conduction state of each SCR 18. The SCRs 18 are driven by pairs into conduction at various times during the cycle of the AC energy from AC source feeding the power transformer, and a resultant DC output voltage is produced.
A generator field current Igf can be achieved by firing the thyristors at a fixed firing angle F (FIG. 2). The thyristors are fired by switching on the thyristor gate current at an instant corresponding to the firing angle. The gate current can have the shape of either a short pulse or a long pulse, either: (i) a slim pulse or (ii) a square wave ending at the next zero crossing of the AC voltage or even lasting until the next thyristor is fired in the following half-wave.
If the load is resistive, Igf becomes zero after the end of the half-wave of the mains voltage and stays zero until another thyristor is fired in the next half-wave. With an inductive load, however, the load forces Igf to continue flowing through the same thyristor and transformer winding 10 even in the next half-wave where it then flows in a direction opposite the mains voltage until another thyristor is fired in that half-wave.
In case of firing angles in the range of 90 to 180 degrees electric, the generator field current Igf becomes discontinuous if the average value Igf is lower than a certain limit. Discontinuity means that the Igf is zero within a certain period of time during each half-wave of the mains ac voltage. Discontinuity is caused by the interaction of two effects. The first is the ripple of the field current which depends on the shape of the field voltage and the values of the field inductivity and resistance. The second is the special behavior of the thyristors to switch off currents below a holding current or latching current.
When operating the thyristor rectifier with an inductive load a discontinuous current, the conventional firing method explained above would be disadvantageous for the following two reasons.